Spawning behavior and embryonic development of the oval squid, Sepioteuthis lessoniana, were observed to clarify the ecological differences by the local population in Akajima and Ishigaki Islands, Southwestern Japan. Allozyme electrophoresis was made for the preliminary investigation of identification of the local population of the squid.1. It has been known that the local population of the oval squid around the coast of Okinawa is classified in the market to several groups by the quality and the price. However, it was very difficult to discriminate the difference among each group conventional morphological ovservations.2. The existence of the spawning cohort which is different from those hitherto known was recognized based on the ecological observations. The newly identified cohort was discriminated from others in respect to the differences of the substratum of egg-laying, number of eggs per egg capsule, egg-laying behavior and the spawning months. Around Ishigaki Island the spawning season of these cohorts never overlaps each other.3. Process of embryonic development between eggs of the 2 spawning cohorts is not different each other except a slight difference of the time of chromatophore appearance on the ventral side of the mantle. However, general morphology and chromatophore pattern of hatchlings of these 2 cohorts do not exhibit spectacular difference.4. For allozyme electrophoresis of the oval squid the muscle of buccal mass and tissue of caecum were found to be useful. Isozymes and polymorphic enzyme of the oval squid were searched in use of starch gel electrophoresis. Out of 26 enzymes studied, several have shown polymorphic zymograms. The studies to search to search for the isozymes and to analyze the local cohorts of the squid in Okinawa are still in the progress.